1. Field of the Invention
The invention relates to an MR method, including:
(a) excitation of nuclear magnetization in an examination zone, subject to a uniform, steady magnetic field, by means of a sequence containing at least one RF pulse, (b) subsequent generation of a read gradient of alternating polarity and simultaneous generation of a phase encoding gradient, (c) acquisition of MR signals occurring after the polarity reversal of the read gradient in order to acquire MR raw data, (d) correction of the MR raw data by means of MR correction data derived from MR signals acquired in the same sequence and with the same temporal variation of the read gradient, however, without phase encoding, and (e) reconstruction of the nuclear magnetization distribution from the corrected MR raw data. The invention also relates to an MR device figured for carrying out such method.
2. Description of the Related Art
It is known that when an MR method in which a plurality of MR signals are successively acquired with an alternating polarity of the read gradient is used, imperfections of the MR system cause phase errors which themselves cause so-called N/2 ghost images.
In order to avoid such artifacts, in an MR method which is known from EPO 490 528 41 in addition to a first sequence, during which the MR raw data are acquired, there is executed a second sequence during which, in the absence of a phase encoding gradient, exactly as many MR signals during the first sequence are acquired as in order to derive MR correction data therefrom. The measuring time is thus doubled. Moreover, the two sequences must be spaced apart in time by an amount which suffices to allow for the nuclear magnetization excited by the first sequence to decay before the second sequence commences. This period of time is of the order of magnitude of one second. A possibly necessary waiting period and the increased measuring period are disadvantageous notably when real-time applications are concerned, for example when the nuclear magnetization distribution is determined continuously in different slices whose orientation changes continuously and which do not extend parallel to one another.
This drawback is avoided by the method which is known from an article by Jesmanowicz et al. entitled "Phase Correction for EPI Using Internal Reference Lines" in SMRM Book of Abstracts August, 1993, p. 1239. Therein, the correction data are derived from two MR signals which successively occur with positive and negative polarity in the course of an EPI sequence when the time integral over the phase encoding gradient has reached the value zero. From these two MR signals there can be derived correction data for the MR raw data acquired with positive polarity of the read gradient on the one hand and with negative polarity of the read gradient on the other hand. However, this method offers satisfactory results only if the phase errors do not vary in time. In the case of a locally inhomogeneous steady magnetic field, however, phase errors occur which vary in time and which cannot be eliminated by means of the known MR method, so that the N/2 ghost images occur as before. Moreover, phase errors induced by eddy currents cannot be detected either.